Communication cables containing optical fibers are generally used to transmit a variety of signals, including voice, video, and data signals. Optical fiber communication cables can be typically grouped into three main categories, which are distinguished by the location of the optical fibers within the cable. All three types of optical fiber cables contain an outer protective covering or polymeric jacket.
In the first category, loose tube fiber optic cables, the optical fibers lie in one or more buffer tubes that are placed about an elongated central strength member. Each of the buffer tubes usually includes a water-blocking material, such as a gel, that prevents moisture intrusion. Where the number of fiber cables is less than the maximum number than can be placed about the central strength member, the loose tube designs may include one or more flexible filler rods. The filler rods, which are typically fabricated from solid or cellular polymers, are wrapped about the central strength member, thereby minimizing the gaps between the central strength member and an outer protective covering or polymeric jacket.
The second and third categories of fiber optic cables are the monotube and slotted core cables, respectively. In monotube cables, the optical fibers are contained within a central buffer or core tube, which contains a water-blocking agent. In slotted core cables, the optical fibers reside in channels or grooves that have been formed on a surface of a rod-shaped polymeric core. The grooves typically follow a helical path along the surface of the core, thereby reducing compressive and tensile forces on the optical fibers whenever the cable is twisted, stretched, bent or compressed. The helical path traversed by the grooves may reverse direction at regular intervals along the cable's longitudinal axis, further reducing such forces acting on the optical fibers. In addition to a central strength member and water-blocking agent, which is disposed in each of the grooves, slotted core cables usually include a buffer tube that covers the slotted core.
Each of the fiber optic cables—loose tube, monotube, slotted core—may include other components, including reinforcing yarns and fibers, rip cords, and additional water-blocking materials (hot melts, water swellable powders, etc.). The fiber optic cables may also include helically wrapped tapes, corrugated armor and similar layers that help protect the optical fibers within the cable.
The buffer tube or core provides the primary protection for the optical fibers. As a result, the buffer tubes usually must exhibit good resistance to compressive, tensile and twisting forces (i.e., crush resistance), while also maintaining a flexibility over a wide range of temperatures. Other desirable properties of optical fiber cables include low cost and low moisture sensitivity, as well as good heat resistance, dimensional stability (e.g., low coefficient of thermal expansion) and chemical resistance.
Conventional buffer tubes are generally made of single layers of polymers and copolymers. Example of these polymers and copolymers include polypropylene (PP) and polyethylene (PE) polymers, copolymers of polyethylene and polypropylene including nucleated polypropylene and polyethylene (n-PP) copolymers, polyamides (PA) such as nylon 12, polybutylene terephthalate (PBT), polycarbonate (PC), fluoropolymers, polybutylene terephthalate, polyester elastomers, acetal resins and the like. Other buffer tube designs use multiple layers of these materials, such as a layer of PBT disposed on a layer of polycarbonate (PC).
Unfortunately, none of these materials is completely satisfactory. For example, PBT exhibits good crush resistance and is perhaps the most widely used material for buffer tubes. However, PBT has marginal flexibility, exhibiting a flexural modulus in excess of about 370 kpsi at room temperature. Though PBT can be treated to make it more flexible, such treatments increase the cost, making it less attractive for buffer tube applications. Additionally, PBT is susceptible to hydrolysis, which results in a loss of strength when exposed to moisture. Polyamides are also susceptible to hydrolysis and tend to be hygroscopic, negatively impacting their mechanical and electrical properties and their dimensional stability.